Heretofore much effort has been devoted to the fabrication of electrical contacts on semiconductor devices, and numerous problems have been encountered in attempting to provide low resistance, solderable ohmic contacts. The fabrication of low resistance, solderable electrical contacts is especially critical in the case of photovoltaic solar cells, since it is well known that the operating characteristics of solar cells are affected by the nature and quality of their front and rear contacts, and also that commercial acceptance of solar cells is conditioned upon the ability to produce reliable, efficient and long-lasting cells and solar cell modules. As used herein, the term "solar cell module" means a plurality of solar cells that are interconnected electrically and physically so as to form a discrete array that provides a predetermined voltage output, e.g., a typical module may consist of 216 cells connected together so as to have a total power output of 220 watts. A selected number of such modules may be connected together to form a solar panel having a desired power output.
One form of photovoltaic solar cell comprises a relatively flat silicon substrate (e.g., a poly-crystalline silicon substrate produced by the EFG crystal growth method) having broad front and back surfaces, a P/N junction formed adjacent its front surface, a first grid-shaped contact or electrode overlying and bonded to the front surface (the "front contact"), and a second flat contact or electrode overlying and bonded to the back surface (the "back contact").
Aluminum, because of its low cost and good electrical conductivity, is the most common material used to fabricate the back contacts of solar cells. It alloys with the silicon so as to form a bond with an acceptable peel strength. However, for purposes of cell interconnections, the back contacts need to be solderable. Unfortunately aluminum tends to form a surface oxide when it is exposed to air. That surface oxide does not prevent the aluminum layer from carrying current generated by the cell; however, it does increase contact resistance to an overlying layer such as a copper conductor, and also inhibits direct soldering.
Heretofore extensive efforts to improve the quality and reduce the costs of manufacturing solar cells have included steps to facilitate the making of solder connections to the aluminum back contacts and otherwise avoiding any deleterious effect from the surface oxide. Typically, the aluminum back contact is made by coating the rear side of a silicon substrate with an aluminum paste (also called an "ink") comprising aluminum particles suspended in an organic vehicle, and then firing the paste in air so as to remove the organic vehicle by evaporation and/or pyrolysis and cause the metal particles to alloy with the silicon substrate and thereby form an ohmic aluminum/silicon contact. Since the air-fired aluminum is not solderable using conventional solders, due to the presence of a surface oxide formed during firing, and since even a nitrogen fired aluminum tends to form a surface oxide when exposed to air, one prior method has involved removing the surface oxide by etching and forming a covering layer of nickel over the fired aluminum layer by an electroless deposition method, and then firing the nickel in a nitrogen atmosphere to bond it to the aluminum. Although the nitrogen-fired nickel is solderable (in contrast to air-fired nickel which is not solderable), a common practice has been to coat the nickel layer with a thin layer of copper or tin which is readily solderable. Copper ribbons or connecting strips are then soldered to the back contact so as to permit connecting the solar cell to other cells or to an exterior circuit. That prior method has a number of disadvantages, including excessive cost and the waste disposal problems associated with a wet electroless plating process.
Another known procedure involves (1) coating the back of the substrate with an aluminum paste so as to form a coating that is discontinuous in the sense that it defines a plurality of openings ("windows") through which the underlying silicon substrate is exposed, and (2) applying a layer of silver to the back of the substrate so as to fill the aforesaid apertures or windows and also overlap a limited amount of the aluminum layer, whereby to form a plurality of silver "contact pads", also called "soldering pads". Tin-coated copper strips are soldered to the silver pads to permit connecting the solar cell to adjacent solar cells. This procedure, described in copending U.S. patent application Ser. No. 561,101, filed Aug. 1, 1990 by Frank Bottari et al for "Method Of applying Metallized contacts To A Solar Cell", offers the advantages that the silver pads provide a plurality of soldering sites for making solder bonds.
Attempts to reduce the cost of manufacturing solar cells have involved investigation and use of a number of coating techniques for applying metallized contacts to the front and back surfaces of the solar cell.
One such known coating technique, commonly identified as screen printing, involves placing a screen having a metallization pattern formed therein on one side of a substrate. A metal screen printing ink is then spread over the metallization pattern in the screen and forced through the screen onto the surface of the underlying substrate by means of a narrow elongate blade which is moved across the screen in direct contact therewith. After removing the screen, the metallized ink is fired to drive off the binder in the ink and cause the metal in the ink to adhere to the substrate. Such screen printing methods are described in U.S. Pat. Nos. 4,293,451 and 4,388,346.
Screen printing may be used for applying contacts to substrates according to this invention, but its use is not preferred for applying back contacts to solar cell substrates which are relatively brittle and have irregular, uneven surfaces, e.g., EFG-grown silicon substrates. Unfortunately, application of metallized inks by screen printing to EFG-grown solar cell substrates can result in significant variation in the thickness of the printed contacts formed from the metallized inks. This variation in thickness is caused by the fact that the surfaces of EFG-grown substrates have undulations or random peaks and depressions with a flatness deviation in the range of 4 to 10 mils. Because the printing screen rests on the high points of an uneven or irregular substrate surface, the thickness of the metallized contacts formed by the screen printing process may vary significantly over the width and length of the metallized contact. Such variation in thickness can result in the excessive use of metal printing ink, thereby adding to the overall cost of the solar cell.
Additionally, if the metal ink is applied in a thickness greater than that required for satisfactory electrical current flow, as occurs with those portions of the screen printed contacts overlying the low spots of the substrate, the substrate may tend to bow as a result of stresses caused by the firing process which bonds the metal ink to the substrate. Such bowing is disadvantageous because it can make the attachment of discrete solar cells to a large solar cell array problematic. Screen printing metallized contacts on a solar cell substrate having uneven surfaces also is not preferred because of the increased risk of substrate breakage. Such tends to occur due to the relatively large forces applied, as measured on a per unit of surface area basis, to the substrate by the narrow blade used to spread the metal ink across the screen. In this connection it is to be appreciated that the typical solar cell substrate is not only brittle but also is relatively thin, usually having a thickness in the range of 0.011" to 0.022".
Techniques such as spraying or evaporative deposition have been considered for applying metallized contacts to a solar substrate. Unfortunately, such techniques involve masking and other limitations. Photolithography may also be used to apply metallized layers in the form of a pattern, such as that of a front surface grid electrode for a solar cell. However, photolithography adds to the time and cost of producing a solar cell substrate.
Most recently the efforts to reduce the cost of manufacturing solar cells has involved the application of pad printing techniques to form contacts. U.S. patent application Ser. No. 561,101, supra, discloses use of pad printing methods to form metal contacts on silicon substrates. The aforesaid application discloses formation of rear contacts by a pad printing method that involves the use of metal printing inks comprising metal particles in a liquid organic vehicle which usually comprises an organic binder and an organic solvent. The aforesaid application discloses a number of metal printing inks that are suitable for forming contacts by the pad printing method. All of the inks are formulated to ensure that (1) the metal particles in the ink will bond to the desired surface, (2) the fired metallized contacts have the desired conductivity, and (3) the inks can be applied easily and repetitively as a thick film having a thickness between about 2 to about 4 mils that will permit the fired contacts to have a thickness ranging from 4 to 10 microns. After the contacts have been applied by the pad printing process, the substrates are fired in a selected atmosphere to drive off the vehicle and securely bond the metal component of the ink to the substrate. The pad printing technique is preferred for the back contacts because it offers significant advantages: it permits formation of contacts at a relatively low cost, with high throughput rates and very low substrate breakage rates.
As noted above, U.S. patent application Ser. No. 561,101 specifically discloses the concept of (1) first forming a back contact on a silicon substrate using the "window" concept, e.g., an aluminum layer with a plurality of apertures ("windows") is formed by pad printing, and (2) then forming silver soldering pads in the windows by pad printing, with the aluminum layer and silver pads being fired to bond them to the silicon substrate.
As heretofore practiced, the "windows" concept of making rear contacts on solar cells using pad printing has been handicapped by the fact that the silver-aluminum contact is somewhat resistive, thus limiting the efficiency of the cell. Virtually the same resistance problem occurs if the aluminum contact has no windows and the silver pads are applied over the aluminum contact. It is recognized that ideally the contact resistance at the silver/aluminum interface should be low enough so as not to limit cell efficiency. Typically this means that the contact resistance should be below about 25 milliohms per silver soldering pad. However, heretofore the contact resistance has been substantially greater.
In the pad printing process described in U.S. patent application Ser. No. 561,101, supra, the aluminum ink is fired in nitrogen. Following that firing, the silver pads are applied and then fired in air. In this "double fired" process, the appearance of the aluminum back contact after the second firing is typically characterized by significant surface oxidation and surface irregularities, e.g., bumps in the aluminum back contact. Also the resulting aluminum back contact is especially sensitive to electrolyte-induced corrosion (determined by immersing the back contacts in 0.01M NaClO.sub.4 solution for 20 hours). The electrolyte-induced corrosion appears to be substantially the same as the aluminum corrosion which occurs after 20 days in a HAST pressure chamber (the term "HAST" stands for highly accelerated stress test) operated at 106 degrees C. and 98% relative humidity. The electrolyte-induced corrosion is characterized by increased silver/aluminum resistance and visual darkening of the aluminum around the silver pads. The surface irregularities hinder cell processing and reduce the mechanical yield during module assembly. The tendency to electrolyte-induced corrosion reduces module life, particularly if no or an imperfect hermetic seal is provided in the backskin of the module that incorporates that solar cell.
Aluminum inks containing a glass frit have been used previously to form aluminum back contacts for solar cells. However, if the aluminum ink contains a glass frit, the contact resistance at the interface of the aluminum and the silver soldering pads is substantially greater than when the aluminum ink contains no glass frit, e.g., over 100 milliohms.